1. Field of the Invention
The present invention relates to an ink ejection device for forming images on a recording medium by ejecting ink droplets from nozzles according to printing commands. The invention also relates to a driving method for the ink ejection device that forms.
2. Description of the Related Art
Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink-ejecting type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on-demand type for ejecting only ink droplets which are used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.
A shear mode type printer using a piezoelectric actuator is one of the drop-on-demand types. Such a printer is disclosed in U.S. Pat. No. 4,879,568. One example of such type of printer is shown in FIGS. 1(a) and 1(b) in which FIG. 1(a) is a cross-sectional view taken along line Axe2x80x94A in FIG. 1(b) an FIG. 1(b) is also a cross-sectional view taken along line Bxe2x80x94B in FIG. 1(a).
As shown in FIGS. 1(a) and 1(b), the shear mode type ink ejection device 600 includes a bottom wall 601, a ceiling wall 602, and elongated shear mode actuator walls 603 sandwiched therebetween. Each actuator wall 603 includes a lower wall 607 adhesively attached to the bottom wall 601 and an upper wall 605 adhesively attached to the ceiling wall 602. The upper and lower walls 605, 607 are polarized in the directions indicated by arrows 609, 611, respectively. Alternating pairs of actuator walls 603 form in alternation ink channels 613 therebetween or spaces 615, which are narrower than the ink channels 613.
Electrodes 619 and 621 are provided on both side surfaces of each actuator wall 603. Specifically, the electrode 619 is provided in the ink channel 613 and the electrode 621 is provided in the space 615. The electrode 621 is also provided on the outer side surface of each of the two outermost actuator walls 603. The electrode 619 is covered by an insulating layer (not shown) to insulate it from the ink. The electrodes 621 are connected to ground 623. The electrodes 619 are connected to a control unit 625 in a form of a silicon chip which applies voltages (driving signals) to the electrodes 619 as will be described later.
A nozzle plate 617 is fixedly secured to one end of the actuator walls 603. The nozzle plate 617 is formed with nozzles 618 at positions corresponding to the ink channels 613. An ink supplying source (not shown) is connected to the other end of the actuator walls 603 through a manifold 626. The manifold 626 includes a front wall 627 formed with openings in positions corresponding to the ink channels 613, and a rear wall 628 for sealing the space between the bottom wall 601 and the ceiling wall 602. Ink from the ink supplying source is supplied to the manifold 626 or common ink chamber and distributed into the respective ink channels 613. The front wall 627 prevents ink from the manifold 626 from entering the spaces 615.
To eject droplets, a voltage from the control unit 625 is applied to the electrode 619 of each ink channel 613. Pairs of the actuator walls 603 deform outward by the piezoelectric shear effect so that the volume of each ink channel 613 increases. In the example shown in FIG. 2, when a voltage E volts is applied to the electrode 619c of the ink channel 613c, an electric field is developed in the actuator wall 603e in the direction indicated by the arrow 631, and an electric field is developed in the actuator wall 603f in the direction indicated by the arrow 632. Because the electric field directions 631 and 632 are at right angles to the polarization direction 609, 611, the actuator walls 603e, 603f deform outward to increase the volume of the ink channel 613c by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber 613c, including near the nozzle 618c. 
Application of the voltage E(V) is maintained for a duration of time T, during which time ink is supplied from the ink supplying source. A pressure wave occurring when the ink is supplied from the ink supplying source propagates in the lengthwise direction of the ink channel 613c. The duration of time T corresponds to a duration of time required for the pressure wave to propagate once in the lengthwise direction of the ink channel 613c. The duration of time T can be calculated by the following formula:
T=L/a
wherein L is the length of the ink channel 613; and
a is the speed of sound through the ink filling channel 613c. 
Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the application of the voltage, the pressure in the ink channel 613c inverts to a positive pressure. The voltage application to the electrode 619c of the ink channel 613c is stopped in timed relation with this pressure inversion so that the actuator walls 603e, 603f revert to their initial shape shown in FIG. 1(a).
The pressure generated when the actuator walls 603e, 603f return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel 613c. This relatively high pressure ejects an ink droplet 26 from the nozzle 618c. 
The inventor of the present invention developed a method for executing a remaining pressure wave canceling operation. In the canceling operation, pressure wave vibrations are generated in the ink of the ink channel 613 after execution of the ejection operation. The canceling operation is executed by applying the voltage E volts to the electrode 619c at a specified time, and subsequently returned to 0 volts in order to increase and then decrease the volume of the ink channel 613. With this canceling operation, not only do the pressure wave vibrations converge at an early stage, thereby preventing unintentional ejection of ink due to residual pressure wave, but also the transition to process the next print command can be performed quicker. As a result, the printing device can form a more faithful image and printing speed can be improved.
However, the speed of sound a described above changes according to the temperature of the ink, as does the actual value of the time T required for a pressure wave to propagate once across the length of the ink channel 613. Therefore, if the canceling operation is executed at a fixed timing regardless of the temperature of the ink, it is possible that the pressure wave vibrations will not be satisfactorily offset. In some cases, an ejection operation corresponding to the next print command may be executed before the pressure wave vibrations have converged, and the ejected ink may spray and scatter or the ink might not even be ejected.
Another problem exits in the remaining pressure wave canceling operation such that reliable cancellation of the pressure wave vibrations in the ink channels cannot be achieved. Because timing for executing the canceling operation has been set to an appropriate value through trial and error. In some cases, the ejection operation for the next print command is executed before the pressure wave vibrations have converged. As a result, the ejected ink can spray and scatter, or sometimes the ink does not eject at all. The canceling operation cannot always reliably cancel the pressure wave vibrations, particularly when driving the ink ejection device 600 at a rapid speed to support rapid printing, in which case the temperature of the ink rises, lowering the viscosity.
In view of the foregoing, it is an object of the present invention to provide an ink ejection device capable of executing a canceling operation to cancel pressure wave vibrations in the ink, regardless of the temperature of the ink.
It is another object of the present invention to provide an ink ejection device capable of forming faithful images by canceling pressure wave vibrations that exist within the ink chamber and capable of increasing the printing speed.
These and other objects of the present invention will be attained by an ink ejection device including nozzles from which ink is ejected; ink channels provided on the back of the nozzles for being filled with ink; an actuator for applying pressure wave vibrations to ink in the ink channels; and a control unit for executing an ejection operation by driving the actuator in response to printing commands to apply pressure wave vibrations to the ink in the ink channels and cause ink to eject from the nozzles, and for subsequently executing a canceling operation by driving the actuator at timings based on the ink temperature in the ink channels to order to eliminate as much as possible the pressure wave vibrations in the ink.
With an invention of this construction, the control unit drives the actuator according to print commands in order to execute the ejection operation in the following way. First, the actuator generates pressure wave vibrations in the ink of the ink channel, which pressure wave vibrations cause ink to be ejected from the nozzle. After the ejection operation, the control unit drives the actuator to execute a canceling operation for sufficiently canceling the pressure wave vibrations in the ink. Accordingly, the pressure wave vibrations are made to converge at an early period to prevent ink from being ejected in a undesirable location as a result of residual pressure wave vibrations, and also to quickly proceed to process the next print command. Moreover, the control unit executes the canceling operation at a timing corresponding to the temperature of the ink in the ink channel. For this reason, the canceling operation can be satisfactorily executed regardless of the temperature of the ink. Therefore, the ink ejection device of the present invention can create an extremely accurate image by preventing ink from spraying and scattering and from not ejecting properly. Further, since the pressure wave vibrations in the ink are converged at an early period, the printing speed can be increased.
The cancellation described above does not need to completely eliminate the pressure wave vibrations. For example, the canceling operation can be configured to only restrain the pressure wave vibrations to a sufficient degree that ink is not ejected from the nozzle.
According to another aspect of the present invention, the actuator varies the volume of the ink channel; and after executing the ejection operation, the control unit drives the actuator to increase and decrease the volumes of the ink channel at timings based on the ink temperature in the ink channel in order to eliminate as much as possible the pressure wave vibrations in the ink.
In the present invention, the actuator described above varies the volumes of the ink channel. Through these volume variations, pressure wave vibrations are generated in the ink, causing ink to be ejected from the corresponding nozzle. The actuator executes the canceling operation by increasing and decreasing the volume of the ink channel. Accordingly, the canceling operation can be extremely proficient in canceling pressure wave vibrations in the ink. Therefore, the ink ejection device of the present invention can even more satisfactorily cancel the pressure wave vibrations in the ink to form an even more accurate image. As a result, the printing speed can be improved even further.
According to still another aspect of the present invention, the control unit drives the actuator to first increase the volume of the specified ink channel and to then decrease the volume of the same to eject ink from the corresponding nozzle, and, following a time interval determined according to both the time required for the ink pressure wave vibrations to propagate one way through the ink channel and the temperature of the ink, subsequently executes a canceling operation by again driving the actuator to first increase and then decrease the volume of the ink channel.
In executing an ejection operation in the present invention, the control unit drives the actuator to first increase the volume of the ink chamber. At this time, the pressure inside the ink chamber decreases, allowing ink to flow into the chamber. Next, the control unit drives the actuator to decrease the volume of the ink chamber. Hence, the pressure inside the ink chamber becomes relatively high, causing ink to be ejected from the corresponding nozzle. After a specified time determined by both the time T required for pressure wave vibrations in the ink to propagate once across the length of the ink channel and the temperature of the ink, the control unit executes a canceling operation by again increasing the volume of the ink channel and then decreasing that volume. Accordingly, the canceling operation can be executed at an appropriate timing based on this time T and the temperature of the ink, and the pressure wave vibration in the ink can be offset very well.
Therefore, the ink ejection device of the present invention is even more proficient in canceling the pressure wave vibrations in the ink and forms an even more accurate image. Further, the printing speed can be increased even more. Since the temperature of the ink is referenced in the present invention, the time T required for pressure wave vibrations in the ink to propagate once across the length of the ink channel can be fixed according to specified temperature ranges, thereby greatly simplifying the process.
According to yet another aspect of the present invention, the control unit executes an ejection operation in response to a one-dot print command and executes a canceling operation after a time interval d2, wherein d2 equals:
2.45 T, when the ink temperature in the ink chamber is between 0 and 10xc2x0 C.;
2.50 T, when the ink temperature in the ink chamber is between 10 and 20xc2x0 C.;
2.55 T, when the ink temperature in the ink chamber is between 20 and 30xc2x0 C.; and
2.60 T, when the ink temperature in the ink chamber is between 30 and 40xc2x0 C.; such that T is the time required for the ink pressure wave
vibrations to propagate one way through the ink channel at room temperature. The control unit performs these operations by driving the actuator to first increase the volume of the specified ink channel and, following an interval of about 1.0 T or an odd multiple thereof, to then decrease the volume of the same to eject ink from the corresponding nozzle. After a time interval of about 0.75 T, the control unit subsequently executes a canceling operation by again driving the actuator to first increase the volume of the ink channel and, following a time interval of about 1.0 T or an odd multiple thereof, to decrease the volume of the ink channel to again eject ink from the corresponding nozzle.
According to this configuration, the volume of the ink channel is first increased and subsequently decreased after approximately 1.0 T or an odd multiple thereof, thereby ejecting ink from the nozzle. After an interval of about 0.75 T, the volume of the ink channel is again increased and subsequently decreased after an interval of approximately 1.0 T or an odd multiple thereof, again causing ink to eject from the nozzle.
Ink ejection is very efficient when the volume of the ink channel is decreased approximately 1.0 T or an odd multiple thereof after the volume of the ink channel has been increased. Since the period of the pressure wave vibrations in the ink of the ink chamber is about 2 T, the timing in which the internal pressure of the ink channel increases after increasing the volume of the ink channel and the timing in which the internal pressure of the ink channel increases due to decreasing the volume of the ink channel coincide, thereby creating a great pressure within the ink channel. Since the control unit executes the ejection operation twice over an interval of about 0.75 T, two droplets of ink are ejected very efficiently for each one-dot print command, thereby forming a rich image.
After completing the ejection operation, the control unit executes a canceling operation after the pulse interval d2, determined according to the temperature of the ink in the ink channel, has elapsed. By executing a canceling operation a pulse interval d2 after the above ejection operation has been completed, pressure wave vibrations in the ink can be very efficiently offset.
Therefore, the ink ejection device of the present invention is even more proficient in canceling the pressure wave vibrations in the ink and forms an even more accurate and richer image. Further, the printing speed can be increased even more.
According to another aspect of the present invention, the control unit increases or decreases the volume of the ink channel by applying a voltage to the actuator, and, moreover, applies the same voltage to the actuator both during an ejection operation and during a canceling operation.
Since the same voltage is applied to the actuator during both the ejection operation and the canceling operation in order to increase or decrease the volume of the ink channel, only one power source is necessary for supplying the drive signal, and, therefore, the construction of the control unit can be simplified. Further, the process for controlling the actuator is simplified by switching the applied voltage off and on. Hence, the construction and control of the ink ejection device can be even more simplified.
According to further another aspect of the present invention, the side walls of the ink channels in the actuator are formed of piezoelectric materials.
Since piezoelectric materials are used to construct the side walls of the ink channels, the volume of the ink channels can be changed by applying voltage to the piezoelectric materials. This type of actuator has a simple construction, is extremely durable, and is inexpensive. Therefore, an invention having this construction can simplify the construction, increase the durability, and further decrease the cost of the device.
The present invention also provides an ink ejection device including nozzles from which an ink is ejected; ink channels provided on the back of the nozzles for being filled with the ink; an actuator for varying volumes of the ink channels; and a driving device for executing an ejection operation by driving the actuator to generate pressure wave vibrations in the ink channels and cause the ink to eject from the nozzles, and for subsequently executing a canceling operation by driving the actuator to first increase and then decrease the volumes of the ink channels in order to eliminate as much as possible the pressure wave vibrations in the ink. The volume increases and decreases of the canceling operation are executed when the pressure wave vibrations cross the center of their vibration or the neutral pressure level, an odd number of times, that is, when the pressure changes from positive to negative.
With the construction described above, the control unit drives the actuator to vary the volume of the ink channels provided on the back of the nozzles, generating pressure wave vibrations in the ink channels and ejecting ink from those channels.
Subsequently, the control unit executes a canceling operation to cancel the pressure wave vibrations in the ink channels. In this canceling operation, the control unit drives the actuator to first increase the volume of the ink channels and subsequently decrease that volume. As a result, the pressure wave vibrations in the ink channels are quickly converged, thereby preventing ink from being ejected in an undesirable location by remaining pressure wave vibrations and quickly proceeding to the ejection operation corresponding to the next print command. Moreover, the control unit increases and decreases the volumes of the ink channels during the canceling operation at points when the frequency wave crosses the center of the vibration an odd number of times, that is, when the pressure changes from positive to negative.
Here, the inventors of the invention studied the relationship between the waveform of the pressure wave vibrations in the ink channels and the time for executing a cancel operation capable of most effectively canceling those pressure wave vibrations. As a result, the inventors discovered that the cancel operation is most effectively executed after the ejection operation when the pressure wave vibrations cross the center of the vibrations an odd number of times, particularly, three times. In this case, the pressure waves in the ink channels are converged at an extremely early period, thereby effectively preventing ink from ejected at an undesirable location and allowing the ink ejection device to proceed very quickly to the ejection process in response to the next print command. Moreover, the canceling operation demonstrates an ability to stabilize the pressure wave vibrations, regardless of various conditions.
Since the waveform shape of the pressure wave vibrations when crossing the center of the vibration a third times is similar to the waveform shape when crossing the center of the vibration in the other odd number of times, it is possible to assume that the effects of executing the cancel operation at these times are approximately the same. In other words, the pressure within the ink channel immediately after completion of the ejection operation has risen near the peak of the pressure wave. Afterward, the pressure wave vibrates periodically and, when crossing the center of the vibration an odd number of times, the pressure wave crosses the center in a declining direction at approximately the same slope.
In the present invention, the canceling operation is executed after the ejection operation and when the pressure wave vibration crosses the center of the vibration an odd number of times. Accordingly, the pressure wave vibrations in the ink channels are converged at an extremely early time, thereby effectively preventing ink from being ejected at an undesirable location and allowing the ink ejection device to proceed quickly to the ejection process corresponding to the next print command. Moreover, the effects of this invention demonstrate reliability. Therefore, the ink ejection device of the present invention forms an accurate image by reliably canceling pressure wave vibrations remaining in the ink channels and improving printing speed.
In the canceling operation described above it is not necessary to completely eliminate the pressure wave vibrations. For example, it is possible to restrain the pressure wave vibrations to the degree that ink is not ejected from the nozzles.
Preferably, the odd number of times is set to three. With this configuration, the invention can reliably cancel pressure wave vibrations. Moreover, it is possible to converge the pressure wave vibrations at an earlier time than when the above-described odd number of times is set to five, seven, or more. Accordingly, an even more accurate image is created by reliably canceling pressure wave vibrations at an even earlier time, thereby improving the print speed even further.
According to another aspect of the present invention, the control unit repeatedly executes the ejection operation a plurality of times in response to a one-dot print command and executes the canceling operation after all ejection operations are completed for the one-dot print command.
With the configuration described above, a plurality of ink droplets is ejected in response to a one-dot print command, thereby forming a richer image. By repeatedly executing the ejection process a plurality of times, the pressure wave vibrations in the ink channels become more complex, making it more difficult to set an appropriate timing for executing the cancel operation. In the present invention, this problem is overcome by executing the canceling operation at a time when the pressure wave vibrations have crossed the center of the vibration an odd number times, as described above. As a result, a timing for executing the cancel operation can be easily set. Moreover, it is possible to reliably cancel the pressure wave vibrations. Therefore, an even richer image can be formed, and costs for developing the ink ejection device can be further decreased by simplifying the settings for the device.
According to another aspect of the present invention, the time that elapses during the canceling operation after the volume of the ink channel is increased and until the volume is decreased is about 0.3 to 0.7 or about 1.3 to 1.7 times the length of time required for a pressure wave to propagate once across the length of the ink channel.
The inventor of the present invention conducted an experiment in which the time interval (hereinafter referred to as the xe2x80x9cpulse width Wcxe2x80x9d) in the cancel operation after the volume of the ink channel is increased until the volume of the ink channel is decreased was changed to various lengths, and studied the values of the pulse width Wc capable of most effectively canceling pressure wave vibrations in the ink channel. As a result, the inventor discovered that the pulse width Wc should be set to 0.3 to 0.7 or 1.3 to 1.7 times the time required for a pressure wave in the ink to propagate one direction along the ink channel (hereinafter referred to as the xe2x80x9ctime Txe2x80x9d).
It is presumed that Wc is best set to these values for the following reason. When Wc=1.0 T, the peak of the pressure wave vibrations generated by increasing the volume of the ink channel and the increase in pressure generated by the decrease in the volume of the ink channel combine to eject ink from the nozzles. When Wc=2.0 T, the pressure wave vibrations caused by increasing the volume and the pressure wave vibrations generated by decreasing the volume cancel each other out, inviting the same effect as when not executing a canceling operation. Accordingly, by setting the pulse width Wc to a value between these values, such as 0.3 to 0.7 T or 1.3 to 1.7 T, it is thought that the canceling operation can be effectively executed.
Since in the present invention Wc is set at approximately 0.3 to 0.7 T or approximately 1.3 to 1.7 T, the canceling operation can be very effectively executed. Therefore, an even more accurate image can be formed by more effectively canceling the pressure wave vibrations, and the printing speed can be improved even further.